Control of a microgrid in islanded mode and Grid-connected modes

Abstract

Microgrids are small electrical power distribution networks that can be connected to a main utility power grid, or can operate in disconnected or islanded mode. They offer a potential solution to the world’s reliance on fossil fuel sourced electrical energy generation. They can improve energy security and reliability for customers connected to the microgrid. Microgrids usually incorporate distributed energy sources, including renewable sources. Many of the renewable sources are interfaced with power electronics which must be able to operate in conjunction with traditional forms of generation such as synchronous generators. Control of power electronic interfaced energy sources can pose challenges for coordinated control of the microgrid as a whole. The control systems main function is to ensure supply to critical loads is maintained, particularly when in islanded mode. This study investigated several strategies for control and management of a microgrid, including taking over voltage and frequency control in islanded mode.

The distributed nature of energy sources that usually makes up a microgrid favours a control system with minimal communication infrastructure and that for the most part operates autonomously in both a grid connected and islanded mode. The current study investigated the general concepts for control of both centralised and decentralised configurations of a microgrid and the problems associated with each. In-depth investigations and simulations were carried out on two decentralised control strategies, a pure droop control method and an angle-frequency droop control method.

The pure droop controller had the ability to autonomously perform equal power sharing and maintain stability in islanded mode of operation, but resulted in permanent steady state frequency offset. The angle-frequency droop also operated autonomously but with improved power sharing and frequency regulation.

The investigation used the MATLAB® environment to perform calculations and carry out simulations of the proposed systems over a range of grid connected and islanded mode scenarios. Performance measures such as power sharing accuracy, disturbance transient behaviour and islanded-grid connection transition were used to assess the suitability of each control scheme.

The P-f droop control has been widely reported on and has been proven to work over a range of conditions. The angle-frequency droop is a new proposal to improve the performance of a microgrid in islanded mode. Results demonstrated that both methods of control performed well in islanded mode. The angle-frequency droop had a slight increase in power sharing accuracy and superior frequency regulation. However it would appear that several design flaws may need addressing before the angle-frequency droop can be implemented as a truly decentralised topology.